a) Field of the Invention
The present invention relates to a hydrodynamic bearing apparatus, in which a dynamic pressure is generated in a lubricant fluid and thereby a shaft unit and a shaft fitting unit are relatively and rotatably supported, and its manufacturing method.
b) Description of the Related Art
Various proposals have been made in recent years for hydrodynamic bearing motors which rotate various rotary disks such as polygon mirrors, magnetic disks, optical disks, etc. In such hydrodynamic bearing apparatus, a hydrodynamic bearing surface on the shaft unit side and that on the shaft fitting unit side are formed to face each other in the radial direction with a predetermined gap. A hydrodynamic bearing portion is created in the facing gap. Hydrodynamic pressure generating grooves are produced on either one of the hydrodynamic bearing facing surfaces. The lubricant fluid injected in the hydrodynamic bearing portion, such as air or oil, is pressured by a pumping action of the hydrodynamic pressure generating grooves during the rotation, and by the dynamic pressure of the lubricant fluid the shaft unit and the shaft fitting unit are relatively and rotatably supported.
Generally, the surface of one of the hydrodynamic bearing surfaces of the shaft unit and shaft fitting unit in such a hydrodynamic bearing apparatus is coated with a lubricant coating material (see FIG. 8), and the other surface is plated (see FIG. 10). Thus, two types of coatings are cooperatively used.
For example, to coat the lubricant coating material on the shaft fitting unit, a blank 1 of the shaft fitting unit as illustrated in FIG. 8 (a) is first formed of aluminum or aluminum alloy by molding or die casting.
Then, as illustrated in FIG. 8 (b), a base treatment 2 with, for example, chromadization or anodic oxidization, is given to improve resistance and coating contact. Also, a masking 3 is given on a portion of the outer surface of the blank 1. As illustrated in FIG. 8 (c), a lubricant coating material 4 containing PTFE (polytetrafluroroethylene), for example, is coated by spray, etc. over the inner surface of the blank 1 and dried, followed by three to five repetitions of spraying for thick coating. Such a thick coating is given because of the uneven thickness of the coating caused by foaming or dripping of the coating material. A material having such uneven coating is given a lathe 5, as illustrated in FIG. 8 (d), and finished so that the thickness of the coating material 4 becomes about 15 micron to obtain a precise inner diameter. Same is for applying the lubricant coating material on the shaft unit.
For coating the lubricant coating material on the shaft unit, supposing that the shaft unit has hydrodynamic pressure generating grooves, the process is as illustrated in FIG. 9. For cutting the hydrodynamic pressure generating grooves, a blank 6 of a shaft unit as illustrated in FIG. 9 (a) is first formed of aluminum or aluminum alloy by lathe or die casting. As FIG. 9 (b) shows, a cap 8 is fitted to a chuck portion 6a of the blank 6 and then, a base treatment with, for example, chromadization or anodic oxidization (alumite method) is carried out to improve resistance and coating contact.
As illustrated in FIG. 9 (c), the lubricant coating material 4 containing PTFE (polytetrafluroroethylene), for example, is applied by spray, etc. over the outer surface of the blank 6 and dried, followed by three to five repetitions of spraying for thick coating. Such a thick coating is given because of the uneven thickness of the coating caused by foaming or dripping of the coating material. A material having such uneven coating is given a lathe 5, as illustrated in FIG. 9 (d), to roughly obtain the outer diameter, and then, hydrodynamic pressure generating grooves 9 are produced on the outer circumferential surface by machining, as illustrated in FIG. 9 (e).
Finally, as FIG. 9 (f) shows, a lathe 5 is given again to finish the blank such that the thickness of the coating material 4 becomes about 15 micron to obtain a precise outer diameter as well as to remove burr produced during the grooving. Same is for grooving the shaft fitting unit that is a counterpart of the shaft unit.
For plating the hydrodynamic bearing surface on the shaft unit, hydrodynamic pressure generating grooves are formed prior to plating when the bearing surface has the grooves. In other words, as illustrated in FIG. 10 (b), a masking printing 7 is carried out on the portion of the blank 6 illustrated in FIG. 10 (a) other than the hydrodynamic pressure generating grooves. With caps 8 fitted to the ends, etching is carried out to produce the hydrodynamic pressure generating grooves 9, as illustrated in FIG. 10 (c).
The next step is plating. FIG. 10 (d) shows a prior treatment of degreasing, activation, etc. After this, as illustrated in FIG. 10 (e), the cap 8 is fitted to the edge for zinc substitution on the surface, and then a plating treatment such as electroless nickel plating is carried out as in FIG. 10 (f).
However, such conventional hydrodynamic bearing apparatus and its manufacturing method have the following drawbacks.
The coating process of the lubricant coating material 4 as illustrated in FIGS. 8 and 9 requires time due to the thick coating, and moreover, a finishing process such as lathe 5 is needed to produce the coating of even thickness. In addition, the base treatment should be carried out precisely. If not, the contact of the coating material is degraded, causing expansion of the lubricant coating material coated surface, peeling, and corrosion of the material. In other words, since the conventional manufacturing process requires the strict process management for quality control, the productivity is not high but the manufacturing cost is high.
Also, the plating treatment as FIG. 10 involves many steps (about 50 steps). Thus, the conventional hydrodynamic bearing apparatus has the drawbacks of poor productivity and high manufacturing cost.
In addition, with the plating treatment, the resistance against corrosion is still not enough. Besides, the surface grows granular fracture, resulting in producing rough surface (not smooth) and making it easy to generate abrasion powder at use. Therefore, this can be a big problem for the apparatus which requires cleanness.
Also, the machining of the hydrodynamic pressure generating grooves 9 requires time and a post treatment such as burr removal. Thus, the conventional manufacturing process is complicated, requiring time for manufacturing. Moreover, many expensive devices need to be prepared.
The process may include a step of coating after producing the hydrodynamic pressure generating grooves by etching or machining. However, the same problems as above are accompanied.
A primary object of the present invention is to provide a hydrodynamic bearing apparatus in which the hydrodynamic bearing surface, of high quality and excellent resistance, can be formed on a shaft unit or a shaft fitting unit in a simple process. The present invention also encompasses a method for manufacturing a hydrodynamic bearing apparatus in which the hydrodynamic bearing surface and the hydrodynamic pressure generating grooves of high quality can be formed simply.
In accordance with the invention, a hydrodynamic bearing apparatus comprises at least a pair of hydrodynamic bearing surfaces that face each other in the radial direction which are formed with an outer circumferential surface of a shaft unit and an inner circumferential surface of a shaft fitting unit relatively and rotatably attached to the shaft unit. Hydrodynamic pressure generating grooves of a predetermined shape are produced on either one of the hydrodynamic bearing surfaces of the shaft unit and shaft fitting unit. A lubricant coating is formed by electrodeposition on either of the hydrodynamic bearing surfaces of the shaft unit and shaft fitting unit. The invention also encompasses a method for manufacturing a hydrodynamic bearing.